A magnetic disk is used as a storage medium for a magnetic disk system. The disk includes a plurality of tracks each having a predetermined width which are assigned in the radial direction of the magnetic disk as a recording area for various data. In each track, sectors are assigned at predetermined intervals in the circumferential direction of the track. In each sector and track, servo data representing the position of the track in sector is recorded.
This servo data is recorded on a magnetic disk as a flux reversal pattern obtained by a track or sector address expressed by a binary number to correspond to a change of data of a (1) or a (0). The magnetic disk system reads back the servo data through the magnetic head to perform positioning control of the magnetic head on the magnetic disk.
A conventional circuit for detecting the servo signal in such a magnetic disk system for performing the positional control for the magnetic head is shown in FIG. 15.
Servo data recorded on magnetic disk 1501 as a flux reversal pattern is read back by a magnetic head 1502. The detected servo data is output as an analog servo signal S1 and the servo signal S1 is supplied to an AGC (Automatic Gain Control) amplifier 1503. The AGC amplifier 1503 amplifies the analog servo signal S1 supplied from the magnetic head 1502 and adjusts the level of the servo signal S1 to a predetermined signal level. The servo signal S2 having a level adjusted by the AGC amplifier 1503 is supplied to a high frequency removal filter 1502.
The high frequency removal filter 1504 removes, based on a predetermined and fixed cut off frequency, a high-frequency noise component included in the servo signal S2 supplied from the magnetic head 1502 through the AGC amplifier 1503. A servo signal S3 from which the high frequency noise component is removed by the high frequency removal filter 1504 is supplied to a pulse decoder 1505 and a integral-type gain detector 1506.
The pulse detector 1505 detects in accordance with a predetermined qualification level TL, peak points corresponding to the flux reversal read portions of the servo signal S3 supplied through the high frequency removal filter 1504, and output each peak point as a binary pulse S4. The servo data recorded on the magnetic disk 1501 is recognized as a positioning address of the magnetic head 1502, representing a track, a sector or the like.
The level of the servo signal S3 from which a high frequency noise component is removed by the filter 1504 is detected by an integral-type gain detector 1506, and the servo signal S3 is feed back to the AGC amplifier 1503 as a gain control signal S5. The AGC amplifier 1503 adjusts an application gain for the servo signal S1 read from the magnetic head 1502 in response to the gain control signal S5. In this manner, a level of the servo signal S2 output to the filter 1504 is adjusted to a predetermined level.
Flux reversal patterns recorded on the magnetic disk 1 as servo data are generally recorded, in synchronism with the predetermined write frequency, at any position in the inner most and outer most tracks of the magnetic disk 1501 rotated at a predetermined speed. For this reason, the data recording density is low in the outermost track (OT) of the magnetic disk 1501 and high in the innermost tracks (ID).
The interval between the respective reversal portions of the servo data is large in the outermost tracks of the magnetic disks of 1501 and small in the innermost tracks. For this reason, when the servo data is read out by the magnetic head 1502, the leading and trailing edges of the signal waveform correspond to each flux reversal portion in the outermost tracks are different from those in the innermost tracks. In the outermost tracks, the leading and trailing edges of the signal waveform are sharp and approximate to those in an independent waveform. In the innermost tracks, the waveforms are interfered with each other to dull the total waveform.
Additionally, servo demodulation in disk drives which incorporate embedded servo sectors has traditionally been achieved by first rectifying the servo burst waveform and then performing either an area detection or peak detection on the rectified burst. However, the servo burst waveform may suffer from asymmetry. Asymmetry is when the amplitude of the positive pulses of the servo burst waveform is different from the amplitude of the negative pulses of the servo burst waveform. This may be caused by a MR lead. Thus, if the amplitude is determined based on a zero to peak of the signal, the amplitude will be different for the positive pulse with respect to the negative pulses. This is caused by a level shift in this case upwards along the vertical axis of the entire signal.
FIG. 3 illustrates an asymmetric waveform. If the asymmetric waveform is rectified, the rectified output is a series of pulses with alternating amplitudes which is illustrated in FIG. 4 as curve 400. As illustrated in FIG. 4, if peak detection is used on this rectified asymmetric signal, then the envelop that is output, shown as curve 404, corresponds to the larger of the two alternating amplitudes, which in turn introduces an error into any further calculations, since curve 404 does not take into account the smaller amplitudes.
One solution is illustrated by the curves of FIGS. 5,6 and 7. Here, the asymmetric waveform, curve 500, is separated into positive pulses 600 and negative pulses 700. The positive pulses 600 is peak detected to generate curve 602. The negative pulses 700 are inverted to peak detect curve 702. FIG. 6 illustrates the positive pulse to form the rectified curve 600 of the positive pulses. Curve 602 illustrates the peak detection of these positive pulses. In contrast, the inverted negative pulses are illustrated in FIG. 7 by curve 700 and are peak detected by curve 702. FIG. 9 illustrates on such circuit. As illustrated in FIG. 9, differential signals, signal DP and signal DN are input to a half wave rectifier 930. A differential amplifier 900 controls the output of differential amplifier 904 and the output of differential amplifier 902. More specifically, the output of each of differential amplifier 902 and differential amplifier 904 is controlled by switches which is controlled by differential amplifier 900. The output is sent to peak detecting circuit 920. This circuit 920 detects the peak of the positive peaks of differential signal DP or the peaks of the differential signal DN depending upon which switch is closed and which switch is open and being controlled by differential amplifier 900. However, the circuit shown in FIG. 9 does not eliminate the asymmetry associated with the input signal.